Field of the Invention
The invention relates to envelope tracking modulated power supplies suitable for radio frequency power amplifier applications. The invention is particularly concerned with such power supplies in which a reference signal is used as an input to a low frequency path and a high frequency path, and in which each path generates separate outputs which are combined to form a supply voltage.
Description of the Related Art
Envelope tracking power supplies for radio frequency power amplifiers are well-known in the art. Typically a reference signal is generated based on an envelope of an input signal to be amplified. An envelope tracking power supply generates a supply voltage for the power amplifier which tracks the reference signal.
FIG. 1 shows a prior art envelope tracking (ET) modulator architecture in which a frequency splitter 12 is used to divide an incoming envelope reference signal on line 10 into a high frequency (HF) path signal on line 14 and a low frequency (LF) path signal on line 16. The frequency splitter 12 may include a low pass filter 18 in the low frequency path and a high pass filter 20 in the high frequency path. The signal in the LF path on line 16 is amplified by an efficient switched mode amplifier 22, and the signal in the HF path on line 14 is amplified by a wideband linear amplifier 24. A frequency selective combiner 26 is used to combine the signals in the LF and HF paths after their respective amplification. In FIG. 1 the combiner 26 is illustrated as including a low frequency combining element 28 in the low frequency path, and a high frequency combining element 30 in the high frequency path. A combined signal from the combiner 26 on line 32 provides a feed to a load 34 which for purposes of example is illustrated as a resistor. In a typical application the load is a power amplifier (PA), and the reference signal is derived from an input signal to be amplified by the power amplifier.
An example of a power amplifier system incorporating a supply architecture such as illustrated in FIG. 1 can be found in “Band Separation and Efficiency Optimisation in Linear-Assisted Switching Power Amplifiers”, Yousefzadeh et al, [IEEE Power Electronics Specialists Conference 2006]
FIG. 2 shows an alternative prior art arrangement, in which the frequency selective combiner 26 is an inductor-capacitor (LC) combiner. The low frequency combining element is an inductor 28a, and the high frequency combining element is a capacitor 30a. In this arrangement a feedback path 36 takes a signal from the combiner (or modulator) output on line 32, to the input of the linear amplifier 24. The signal on the feedback path 36 is subtracted from the signal in the high frequency path on line 14 by subtractor 38 to provide an input to the linear amplifier 24. The inclusion of the feedback path 36 achieves improved tracking accuracy compared to the arrangement of FIG. 1.
An example of a power amplifier system incorporating a supply architecture such as illustrated in FIG. 2 can be found in “Efficiency Optimisation in Linear-Assisted Switching Power Converters for Envelope Tracking in RF Power Amplifiers”, Yousefzadeh et al, [IEEE Symposium on Circuits and Systems 2005].
A problem may arise in prior art arrangements such as illustrated in FIG. 1, for example, in that resonance can occur in the output of the switched mode amplifier and/or the combiner at certain frequencies, which can reduce the efficiency and tracking accuracy of the envelope tracking modulated power supply at those frequencies.
It is an aim of the invention to provide an envelope tracking modulated power supply which offers improvements over the prior art, and in particular which addresses the resonance problem.